Apparatus for magnetically coupling an induction heating coil with a ferrous cooking utensil to thereby electromagnetically heat the contents of the utensil have been widely known and used for many years. In such apparatus, the induction coil is usually located below a nonmagnetic cooking surface and an alternating current through the coil causes a continuously changing magnetic field to be generated. The magnetic field extends through the cooking surface to link with the cooking utensil to cause eddy currents in the utensil and allow it to heat up.
Commercial versions of induction cooking apparatus provide for a plurality of cooking areas on a smooth-top cooking surface made from a single continuous rectangular piece of ceramic material. Each designated cooking area on the cooking surface has an induction cooking coil located thereunder so that cooking utensils placed on the designated cooking areas will be linked by the magnetic field generated by the cooking coil.
One problem associated with this construction is cost; the ceramic cooktop is more expensive than a comparably sized sheet steel cooktop typically used in conventional electric or gas ranges.
Another problem is electromagnetic leakage. The electromagnetic leakage problem is aggravated by a variety of factors. One factor is the misalignment of a cooking utensil with the magnetic field generated by the cooking coil. This problem is addressed in the context of a conventional induction cooking construction by providing utensil presence and position detection apparatus which insure that the induction coil is not energized unless the cooking utensil is both present on the cooktop and centered over the induction heating coil.
These sensing arrangements are designed to insure that the high intensity electromagnetic fields which emanate from the induction heating coil are generated only when a utensil is in position and centered over the induction heating coil, thereby limiting the undesirable transmission or leakage of electromagnetic flux into the free space surrounding the cooking appliance. Neither of these approaches, however, addresses the problem of electromagnetic leakage resulting from the high reluctance gap present in the flux path between the edges of the cooking utensil and the flux-shaping coil support structure. This latter situation creates an undesirable condition which results in the leakage of excessive magnetic flux into the space surrounding the cooking surface, which leakage may cause interference with television and radio signals and other communication systems. For this reason, among others, governmental regulating agencies have set limits on the magnetic field leakage of this type attendant to the use of induction heating appliances. Since the intensity of flux leaking into surrounding space increases as a result of operation of an induction heating unit with such high reluctance gaps, it is desirable to provide an arrangement for operation of the unit without such gaps or with a reduced number of them.